Electronic cooking appliance



Sept. 17, 1963 F. H. SAWADA ELECTRONIC cooxmc APPLIANCE 2 Sheets-Sheet 1 Filed Sept. 12, 1960 INVENTOR FRED H. SAWADA QT 5 m2 8 m h 2 J a m D mg was bi m1 3 Ni b QR ms ms Q2 wl mm w EM Sept. 17, 1963 Filed Sept. 12, 1960 PLATE CURRENT F. H. SAWADA ELECTRONIC COOKING APPLIANCE 2 Sheets-Sheet 2 Resonance MP3 T Loaded dIP4 I am dIP2 FREOUENC Y INVENTOR. FRED H. SA WADA ATTYS United States Patent 3,104,304 ELECTRUNIC COOKING APPLHANCE Fred H. Sawatla, Wheaton, Ill, assignor to General Electric Company, a corporation of New York Filed Sept. 12, 196i), Ser. No. 55,326 14 Claims. or. 219-1055) The present invention relates to electronic cooking apparatus, and more particularly to such apparatus provided with an improved arrangement for automatically adjustmg the operating frequency-of the oscillator supplying the high frequency cooking power to the resonance frequency thereof to compensate for changes in the characteristic load impedance of the food being cooked and to counteract any tendency of the frequency of operation of the oscillator to drift from the resonance frequency thereof.

Electronic cooking apparatus conventionally comprises metal wall structure defining a substantially closed cooking cavity, and an oscillator for supplying high frequency power to effect cooking in the cooking cavity, the oscillator having an amplifying device in circuit with a tank circuit including an adjustable inductor and a capacitor in the form of spaced apart plate-s disposed in the cooking cavity and adapted to receive .therebetween the food to be cooked. It has been customary heretofore to adjust the circuit parameters of the oscillator to accommodate the electrical characteristics of a particular item to be cooked, whereby the characteristic load impedance thereof is known, so that the oscillator can be operated at the resonance frequency thereof thus providing for more economic operation of the electronic cooking apparatus.

If the characteristic load impedance of the food being cooked changes materially during cooking there-by to cause frequency drift in the oscillator, or if the food being cooked is not that for which the cooking apparatus has been designed, so that the characteristic load impedance of the food being cooked is substantially dilferent from that for which the cooking apparatus was designed,

then the capacitance forming a part of the frequency determining circuit of the oscillator is changed so that the oscillator operates at a frequency other than the resonance frequency thereof, and the oscillator may even opcrate at a frequency that is not within the range allocated by the Federal Government for operation of the electronic cooking apparatus whereby to render operation of the cooking apparatus objectionable and unlawful. Furthermore, operation of the oscillator at a frequency other than the resonance frequency thereof results in a substantial loss of efliciency of operation thereby rendering the cooking operation more costly and more time consuming than if the cooking operation were carried out at the resonance frequency of the oscillator.

Accordingly, it is a general object of the present invention to provide an electronic cooking apparatus of the character described with an improved arrangement for automatically adjusting the frequency of operation of the oscillator to accommodate differences in the characteristic load impedance of the food being cooked.

Another object of the invention is to provide in electronic cooking apparatus of the character described an improved control circuit responsive to the current flowing in the amplifying device forming a part of the oscillator for automatically adjusting the frequency of the tank circuit so that the oscillator operates at the resonance frequency thereof to counteract any frequency drift of the oscillator.

A further object of the invention is to provide in electronic cooking apparatus of the character described an improved circuit control arrangement for the oscillator that is effective to change the inductance of the inductor forming a part of the tank circuit of the oscillator in response to increases in the current flowing through the "ice 2 amplifying device of the oscillator thereby to compensate for the differences in the characteristics of the load impedance of the food being cooked.

A still further object of the invention is to provide in electronic cooking apparatus of the character described an improved control circuit for the oscillator :that is operative to change the frequency of operation of the oscillator from a frequency either above or below the resonance frequency of the oscillator, thereby to compensate for diiferences in the characteristic load impedance of the food being cooked and to insure operation of the cooking apparatus in an efficient manner.

Further features of the invention pertain to the particular arrangement of the elements of the electronic cooking appliance and of the associated control network, whereby the above outlined and additional operating features thereof are attained.

The invention, both as to its organization and method of operation together with further objects and advantages thereof will best be understood by reference to the following specification taken in connection with the accompanying drawings, in which:

FIGURE 1 is a schematic diagram of the electronic cooking appliance and the control network therefor; and

FIG. 2 is a graph showing the relationship between the frequency of operation of the oscillator of the electronic cooking appliance of FIG. 1 and the plate current flowing through the amplifying device forming a part of the oscillator for the electronic cooking device.

Referring now to FIG. 1 of the drawings, there is illustrated an electronic cooking appliance 1t embodying the features of the present invention and comprising a range 11 of any suitable construction. As illustrated, the range 11 comprises a metallic enclosure including a housing section 12 provided with an open front and a front door section 13 and defining a substantially closed oven cavity or chamber 14, the door section 13 being mounted adjacent to the lower edge thereof upon the housing section 12 by means of a hinge 21 for movements between a substantially vertical closed position and a substantially horizontal open position. The housing section 12 comprises a metal inner liner l5 and a metal outer shell 16 arranged generally in spaced-apart relation with thermal insulation 17 therebetween, if desired; and similarly the door section 13 comprises a metal inner liner 18 and a metal outer shell 19 arranged in generally spaced apart relation with thermal insulation 20' therebetween, if desired.

High frequency energy in the high frequency band is supplied to the oven cavity 14- from a generator or oscillator 10 0 including a tank circuit having as the principal portion of the capacitance thereof a pair of spaced apart capacitor plates CT which are disposed within the cavity 14 and are mounted upon the housing section 12 by suitable insulators (not shown), the plates CT being adapted to receive therebetween the food or other material to be cooked. Since relatively high potentials are applied to the plates CT, means is provided to insure that the housewife or other user of the range 11 does not come in contact therewith while high potentials are still thereon and to this end a safety and grounding device 49 is provided for each of the capacitor plates CT, the grounding devices 40 being automatically operable to ground the plates CT when the door section 13 is opened such as by grasping the handle 22 thereon. Each of the grounding devices 40 includes an elongated handle 41 adapted to contact the inner liner 18 when the door section 13 is in the closed position, the handle 41 being pivoted upon a post 43 and having rigidly connected thereto a blade 42 movable therewith. When the door section 13 is in the open position, the blade 4-2 is movable to contact with spring contactors 44 mounted on the Patented Sept. 1?, 1963 associated capacitor plates CT to make electrical contact therewithunder the urging of a spring 45 interconnecting the handle 41 and the adjacent inner liner 15. Upon movement of the blade 42 into engagement withthe contactors 44, an electrical connection is made from the associated capacitor plate CT through the contactors 44 and the blade 42 to the metal inner liner which is further connected by a conductor 24 to the outer metal liner 16 which is in turn grounded through a conductor 50. Accordingly, upon opening of the door section 13, the springs move the handles 41 to elfect engagement of the blades 42 with the contactors 44 thereby to ground the capacitor plates CT and to remove any dangerous potentials therefrom; upon closing of the door section 13, the inner liner 18' thereof contacts the handles 41 to pivot the handles and the associated blades 42 to the position illustrated in FIG. 1 of the drawings thereby to move the blades 42 away from the contactors 44 and to remove the grounding connection from the capacitor plates CT.

Another safety device is provided in the form of a door switch 30 disposed in the main power supply line, the door switch 30 including a pair of contacts 31 and 32 operable by means of an arm 33 into the closed connecting position thereof when the doorsection '13 is closed and being automatically operable into the open position thereof when the door section 13 is'open, thereby to remove all operating potential from the electronic cooking appliance 10 when the door section 13 is in the open position thereof. 7

The power for operating the electronic cooking appliance 10 including the control network therefor is derived from a main supply including a pair of conductors and V 51 which m ay have impressed thereon a 236 volt A.C.

single phase voltage,.the' conductor 50 preferably being grounded. A switch 52 connects the conductors 50 and 51 to conductors 53 and 54, respectively, the conductor 54 connecting with a fuse F1, which in turn connects with a conductor 55.. The conductor 53 is connected to one of the input terminals 62 of a power supply source 60 and the conductor 55 connects through the door switch 30 to a conductor 56 which in turn is connected to the other input terminal 6-1 of the power supply 60.

The oscillator further includes an amplifying device 101 which has been illustrated as being a three element thermionic tube, it-being understood that other suitable amplifying devices may be substituted therefor. The tube 101 includes a directly heated cathode 102, a plate 103, and a control grid 104; power for operation of the cathode 102 is provided from a transformer T1 including a primary winding 105 and a secondary winding 106, one of the terminals of the primary winding 105 being connected to ground through the conductor 50, and the other terminal thereof being connected to the conductor 55 thereby to impress the full'supply voltage across the primary winding 105. The secondary winding 106 has the ends thereof connected to the ends of the cathode 102 by the conductors 107 and 108, respectively, and further is provided with a center tap to which is connected a conductor 109.

The oscillator 100 further includes the tank circuit which in addition to the capacitor plates CT includes the inductor LT which is illustrated in FIG. 1 as a coil having one terminal .thereofconnected to the upper capacitor plate CT by a, conductor 121 and the other terminal thereof connected to the lower capacitor plate CT by means of a conductor 122, the conductors 121 and 122 being shielded by "a shield connected to ground and extending through an aperture 23 in the wall of the housing section 12. One side of the tank circuit 120 is connected by the conductor 121 to one terminal of a DC. blocking capacitor C1 which has the other terminal thereof connected by a conductor 123 to the plate 103 of the tube 101. The conductor 123 is further connected to one terminal of a high frequency choke coil L1 having the other terminal thereof connected by a conductor 65 to one of the output terminals 63 of the power supply 60 whereby to apply a suitable operating potential to the plate 103; the other output terminal 64 of the power supply 60 is grounded through the conductor 50. The other side of the tank circuit is connected by the conductor 122 to one terminal of a DC. blocking capacitor C2 having the other terminal connected to the conductor 119 that connects to the control grid 104 of the tube 101. The inductor LT further has a tap thereon which is connected through the conductor 50 to ground. The control grid 104 of the tube 101 is further connected to one terminal of an inductor L2, the other terminal of the inductor L2 being connected to a conductor 1118 connected to one terminal of a resistor R1 having the other terminal thereof connected to the conductor 109. There further is provided in the cathode circuit of the tube 101 a resistor R2 having a movable contact 111 thereon connected to ground through the conductor 50, one end of the resistor R2 being connected to the conductor 109. The conductor 109 further is connected to a movable switch contact and cam follower 112forming a portion of a pair of switch springs S1 and a pair of switch springs S2. The stationary contact of the switch springs S1 is connected by a conductor 113 to one terminal of a coil 115 and the other terminal of the coil 115 is further connected by a conductor 114 to a stationary contact of the switch springs S2. The coil 115 has a center tap thereon connected by the conductor 50 to ground and dividing the coil 115 into two sections 115A and 11513.

Means is provided to vary the inductance of the inductor LT in the form of a hollow cylinder having an upstanding cylindrical -wall 126 and a bottom wall 127 closing one end of the cylindrical wall 126 and made of a conductive and non-magneticmaterial such as copper. The copper cylinder 125 acts as a shorted secondary of the inductor LT and an increase in the eddy current shielding thereof is eifective progressively to lower the inductance of the inductor LT as the cylinder 125 is moved downwardly as viewed in FIG. 1 to increase the coupling thereof with the inductor LT and a decrease in the eddy current shielding thereof is effective progressively to increase the inductance of the inductor LT as the cylinder 125 is moved upwardly as viewed in FIG. 1 to decrease the coupling thereof with the inductor LT. It is to be understood that other shapes can be used instead of the cylinder 125 as, for example, a flat plate having the diameter thereof less than the diameter of the opening in the inductor LT and disposed perpendicular to. the longitudinal axis of the inductor LT. The bottom wall 127 of the cylinder 125 has an aperture therein substantially centrally thereof to receive therethrough an upstanding rod 128 which is preferably formed of a good electrical insulating material such as the rigid thermo setting plastic developed specifically as an electrical insulating material for ultra high frequencies and sold under the trademark Rexolite by The Rex Corporation of West Acton, Massachusetts. Preferably the walls of the aperture in the bottom Wall 127 are threaded and cooperate with mating threads on the upper end of the a rod 128 whereby to permit adjustment of the position of the cylinder 125 along the length of the rod 128 and there is further provided a pair of lock nuts 129 threadedly engaging the threaded portion of the rod 128 and disposed above and below the wall 127 to lock the cylinder 125 in the desired adjusted position upon the rod 128. The lower end of the rod 128 carries an iron piece 130Which is disposed within the coil 115 and is normally held in the electrical center thereof.

Mechanism is provided simultaneously to open the resses a relatively higher position disposed between adjacent pairs of the cam surfaces 132 whereby the cam surfaces 132 and 133 alternate upon the periphery of the cam 131, each of the cam surfaces 132 and 133 having the same arcuate extent, whereby 45 degrees rotation of the cam 131 Will move the follower 112 from a cam surf-ace 133 to a cam surface 132 or from a cam surface 132 onto a cam surface 133, the switch springs S1 being closed when the follower 112 is upon a cam surface 133 and the switch springs S2 being closed when the follower 112 is upon a cam surface 132.

A shaft 134 connects the cam 131 to a ratchet wheel 135 having eight teeth thereon corresponding to the eight cam surfaces 132 and 133 on the earn 131 and also having a cooperating pawl 136 contacting the teeth thereon. The pawl 136 is pivotally mounted on an arm 137 which in turn is pivotally mounted on a stationary support as at 138. Also mounted on the arm 137 is a magnetic armature 142 adapted to cooperate with an electromagnet 141 and having one end thereof connected by a spring 14-3 to a stationary point whereby the arm 137 and the armature 1 12 are biased to the right as shown in FIG. 1 against the action of the magnet 14 1. A stop 144 limits the angular extent of the rotation of the arm 137 in a clockwise direction when the arm 142 moves away from the magnet 141. The magnet 14 1 includes a coil 1413 having one terminal thereof connected to the supply con ductor 55 and the other terminal thereof connected to a conductor 145 which connects to a pair of normally closed switch springs S4 which in turn are connected by a conductor 146 to a second pair of normally closed switch springs S3 which in turn are connected by the conductor 511 to ground.

The rod 123 is normally held in a rest posit-ion by a resilient member or spring 1 17 which has one end thereof fixed and the other end thereof extending between a pair of guides 148 and 149 on the rod 123, whereby the spring 147 serves to position the rod 128 at a predetermined point or rest position when the various circuits are de-energized, movement of the rod 125 and the associated parts upwardly and downwardly being against and being resisted by the action of the spring 147. The spring 147 further carries a pair of actuators 1513* and 151 associated respectively with the switch springs S3 and S4, whereby the switch springs S3 are opened upon movement of the rod 128 upwardly a predetermined distance and the switch springs S4 are opened upon movement of the rod 123 downwardly a predetermined distance.

When the cooking appliance 11B is not in use and the switch 52 is open to remove all operating potential therefrom, the tube 1%1 is non-conducting and there are no oscillations produced by the oscillator The rod 128 carrying the cylinder 125 is held in the rest position thereof by the spring 147, the iron member 136 is disposed at the electrical center of the coil 115, and the armature 142 is spaced from the magnet 14-1 to the right under the urging of the spring 143. The position of the cylinder 125 upon the rod 128 is adjusted so that the rod 128 will remain in the rest position thereof when the frequency of operation of the oscillator lttd is at the resonance frequency thereof in the high frequency range and when a predetermined load is disposed between the capacitor plates CT, the load reflecting a predetermined and characteristic load impedance into the tank circuit 120. The position of the contact 111 upon the resistor R2 can also be adjusted so that the proper amount of current flows through the path that can be traced from ground to the center tap of the coil 115, through the conductor 114-, for example, through the switch springs S2, and through the conductor 1G9, whereby subsequently to achieve operation of the oscillator 1110 at the resonance frequency thereof.

All of the parts having high frequency energy thereon are disposed within a shield 160 which is grounded through the conductor 50 whereby to prevent unauthorized radiation from the oscillator 1011 at the high frequencies used.

To begin operation of the cooking appliance 10, the switch 52 is closed whereby to apply operating potential to the conductors 53 and 5d and if the door section 13 is closed thereby closing the door switch 30, the power supply 611 is provided with operating potential and in turn supplies the required operating potentials for the oscillater 16% and the associated control network. The oscillator 1 3d operates at a frequency determined by the circuit parameters thereof and primarily by the values of the capacitance between the capacitor plates CT and the inductance of the inductor LT. With the parts in the position illustrated in FIG. 1 of the drawings after closure of the switch 52 and particularl with the contact 112 in engagement with the cam surface 132 on the cam 131, the circuit for flow of current through the amplifying tube 101 can be traced from the output terminal 63 of the power supply 66 through the conductor 65 and the inductor L1 through the tube 101 from the plate 103 to the cathode thereof, through the conductors 107 and 103 in parallel through the secondary winding 1% of the transformer T1 to the conductor 169. There are two parallel paths from the conductor 1419 to ground, the first path to ground being through that portion of the resistor R2 between the conductor 1159 and the contact 111 and the second path to ground being through the switch springs S2, the conductor 114, the coil section 1 153 and the conductor 51?.

Connected in parallel with the amplifying tube M1 is a tank circuit 12% including the inductor LT and the capacitor plates CT disposed in the heating cavity 14, one side of the tank circuit 121 being connected to the plate 13 through the DC. blocking capacitor C1 and the other side of the tank circuit 126 being connected to the control grid 1114 through the DC. blocking capacitor C2, with the tap on the inductor LT grounded, whereby the tank circuit 12d comprises the principal frequency determining element in the oscillator 1%. In order to provide the necessary feedback from the tank circuit 12s) to the control grid 1114, the connection is made through the conductor 11'? whereby a portion of the potential on the inductor LT is returned to the control grid 1% and developed across the inductor L2 and the resistor R1 in series to ground through the conductor 169, one of the switch springs S1 or S2 (the switch springs S2 as illustrated in FIG. 1), and one section of the coil 115 (the section 115B as illustrated in FIG. 1) to the grounded conductor 51 When the parts are connected as illustrated in FIG. 1 and as described above, the oscillator 161i is adapted to oscillate at a predetermined resonance frequency and to deliver high frequency cooking energy to the cooking cavity 14 to cook material disposed between the capacitor plates CT. When the oscillator is operating at the resonance frequency thereof with no load between the capacitor plates CT, a predetermined current flows through the amplifying tube 101 and supplies the energy necessary to sustain oscillations and to overcome power losses in the circuit. Upon placement of a load between the capacitor plates CT, power is withdrawn from the oscillator 1% and from the power supply on and, accordingly, the plate current through the amplifying tube 101 increases, the increase in plate current representing the power withdrawn in the cooking operation. Placement of the material to be cooked between the capacitor plates CT further serves to change the operating frequency of the oscillator .100. There is shown in FIG. 2 of the drawings the relationship between the frequency of the oscillator 1G1 and the plate current through the tube 161, the relationship with no load between the capacitor places CT being drawn in solid lines and the relationship when there is a predetermined load having a characteristic load impedance between the capacitor plates CT being drawn in dashed lines. The resonance frequency has been designated FR with frequency increasing toward the right whereby the frequency F1 is a frequency below the resonance frequency PR and the frequency F2 is a frequency above the resonance frequency FR.

Assuming that the oscillator tilt) is operating at the resonance frequency thereof with no load between the capacitor plates CT, the oscillator ltitl will operate at the frequency FR indicated in FIG. 2 and the plate current through the tube fill will be 1P1. As the circuit parameters of the oscillator change to cause operation thereof at a frequency less than the frequency FR such as, for example, the frequency Fl, then an increase in the plate current through the tube 101 will occur whereby to provide a plate current 1P2 which is greater than the plate current 1P1 by the increment rill-ll corresponding to a frequency change of dFll. When the change in frequency is induced by the placing of a load to be cooked between the capacitor plates CT, the increase in current flowing through the amplifying tube 101 can be utilized to tend to adjust the frequency of operation toward the resonance frequency FR, whereby the increase in current caused bythe withdrawal of energy from the tank circuit 12d can be utilized for stabilizing the operating frequency of the oscillator 1% at the resonance frequency FR thereof and thereby to accommodate the change in the capacitance of the tank circuit 1% occasioned by the placing of the load between the capacitor plates CT. This is accomplished primarily by movement of the cylinder 125 in response to the change in the current flowing through the amplifying tube 101 and particularly through the cathode circuit thereof. A portion of the increased current through the tube iiil will also flow through the coil section 115B whereby to create a field therein tending to move the iron member 139 downwardly toward the electrical center of the coil section 1153. Such movement of the iron member 130 will pull the rod 128 downwardly against the action of the spring 147 thereby to lower the cylinder 125 and to increase the coupling between the cylinder 125 and the inductor LT. Since the frequency of operation of the oscillator 16% is inversely proportional to the square root of the inductance of the inductor LT, a decrease in the inductance thereof occasioned by the downward movement of the cylinder "125 will increase the frequency of operation of the oscillator 1%. Accordingly, it will be seen that the increased current through theamplifying tube 101 occasioned by the heating losses in thematerial between the capacitor plates CT can be utilized for stabilizing the operating frequency of the oscillator 109 at the resonance frequency PR and more specifically to shift the frequency of operation toward the resonance frequency to compensate for changes in the capacitance between the capacitor plates CT occasioned by the introduction of the material to be heated between the capacitor plates CT. More specifically, the load introduced between the capacitor plates CT ordinarily tends to increase the capacitance thereof thereby to decrease the frequency of operation of the oscillator'ltltl since the frequency of operation is inversely proportional to the square root of the capacitance of the oscillator circuit. Referring to FIG. 2 of the drawings, assuming that the load introduced within the cooking cavity 14 between the capacitor plates CT changes the capacitance of the oscillator 1% so that the oscillator operates at the frequency F1 and further assuming that the relationship between the frequency of operation of the oscillator 100 and the plate current through the amplifying tube 'lltll with the load between the capacitor plates CT is that illustrated by the dashed curve of FIG. 2, then the increase in current dIPl flowing through the coil section 115B will pull down the cylinder 125 to increase the coupling thereof with the inductor LT and change the inductance of the oscillator 1% so that it operates substantially at the resonance frequency FR on the curve labeled Loaded whereby a maximum efiiciency the oscillator 1% thereby to shift the frequency of opof transfer of energy from the oscillator 1% to the material between the capacitor plates CT is realized, the energy transferred being a function of the increase in plate current dlPl. Any tendency to change the frequency of operation of the oscillator 1% or for the frequency'of operation to drift will be resisted also by the action of the coil 115 drawing the cylinder 0125 into closer coupling with the inductor LT so that operation of the oscillator 184? is stabilized at substantially the resonance frequency FR thereof.

As has been noted above, the various circuit parameters including the adjustment of the contact 1111 on the resistor R2 and the placement of the cylinder 125 on the rod i28'are chosen and adjusted to accommodate between the capacitor plates CT a usual load having a predetermined load impedance that isreflected into the tank circuit 12%? to effect the operating frequency of the oscillator 1%. In certain instances the change in capacitance between the capacitor plates CT caused by the introduc tion of a load therebetween may be such as to cause operation of the oscillator 1% at a frequency higher than the resonance frequency FR thereof, such as, for example, at the frequency F2. The increase in current flow through the amplifying tube 1M and through the coil section 115B will again tend to increase the frequency of operation of eration of the oscillator 16b to even a higher value such as, for example, that indicated at P4- whereby the action of the coil 1153 and the cylinder 125 is in a direction to aggravate the departure of the operating frequency from the resonance frequency FR. In order to accommodate the introduction of such load between the capacitor plates CT a circuit is provided to change the effect of an in crease in the current flowing through the amplifying tube ftil upon the frequency of the operation of the oscillator itiil and more particularly to reverse the action thereof so that an increase in current tends to cause a decrease in the frequency of operation of the oscillator 1%. By opening the switch springs S2 and closing the switch springs 81, the desired relationship between the plate current and the frequency of operation of the oscillator wil is-obtained, the cathode circuit from the conductor 1439 now being through the switch springs S1, the conductor 113, the coil section A and the conductor 50 to ground. The current flowing through the coil section 115A creates a field therein tending to move the iron member 139 upwardly toward the electrical center of the coil section 115A. Such movement of the iron member 130 will raise the rod 128 upwardly against the action of the spring 147 to raise the cylinder 125 and to decrease the coupling between the cylinder 125 and the'iuductor LT. Reducing the coupling between the cylinder 125 and the inductor LT serves to increase the inductance in the tank circuit and the oscillator 1th and since the frequency of operation of the oscillator 10th is inversely proportional to the square root of the inductance of the inductor LT, an increase in the inductance thereof occasioned by the upward movement of the cylinder will decrease the frequency of operation of the oscillator ftltl. Thereby the increase in plate current dIPZ will now cause the frequency of operation of the oscillator ltltl to be shifted downwardly toward the resonance frequency FR thereof and drawing a plate current 1192 on the curve labeled Loaded. Accordingly, operation of the oscillator ltlb will tend to be stabilized at substantially the resonance frequency FR thereof.

Inasmuch as the characteristic load impedance of the material placed between the capacitor plates CT may be either of the character which tends to cause operation of the oscillator 1% at the frequency F1 or the character that tends to cause operation of the oscillator flit) at the frequency F2, mechanism must be provided to place the proper one of the coil sections 115A and 1153 in the cathode circuit of the amplifying tube ltil as required and to this end the switch springs S1, S2, S3 and S4, the cam 7 spring which is 131, the ratchet wheel 135 and the magnet 141 have been provided. Assuming that the parts are in the position illustrated in FIG. 1 at the beginning of a cooking operation and that a material is placed between the capacitor plates CT which tends to cause operation of the oscillator 1% at the frequency F2, then a current IFZ will flow through the amplifying device M1 and a portion of that current will flow through the coil section 115B, thereby to move the cylinder 12.5 downwardly decreasing the inductance of the inductor LT and raising the frequency of operation of the oscillator 1%. An increase in the frequency of operation of the oscillator 1% can change the frequency of operation toward the resonance frequency FR and in fact the increase in the frequency of the operation of the oscillator res will cause a further increase in the plate current, the action being cumulative and regenerative so that eventually a current 1P3 will be drawn through the tube Till when the frequency of operation of the oscillator 1% reaches the frequency F 5. At this time the portion of the current 1P3 flowing through the coil section 1158 is sufficient to move the spring 147 and the actuator 156i downwardly a distance that will open the switch springs S4 thereby interrupting the circuit to the magnet coil 14h. Heretofore the magnet coil 14% has been energized by a circuit that can be traced from the main line conductor 51 through the switch 52, the conductor 54, the fuse F1 and the conductor 55 to one terminal of the coil Mil and from the other terminal of the coil Mil through the conductor 145, the switch springs S4, the conductor M6, the switch springs S3 and the condutcor 50 to ground. The current flowing in the coil 14% serves to draw the armature 14 2 to the right as shown in FIG. 1 thus storing energy in the under tension. Therefore opening of the switch springs S4 de-energizes the coil 14b and releases the armature 142 and the energy stored in the spring 143 will move the arm 137 to the right and in a clockwise direction, thereby to cause the pawl 36 to move the ratchet wheel 135 in a counterclockwise direction 45 degrees and through the shaft 134 to move the cam 131 in a counterclockwise direction 45 degrees. This places the cam follower 112 on a cam surface 133 thereby to open the switch springs S2 and to close the switch springs S1 whereby to remove the coil section 1153 from the cathode circuit of the amplifying tube H31 and to substitute therefor the coil section 115A. The increase in plate current (dIP4 in FlG. 2) acting through the coil section 115A will now raise the rod 128 and the cylinder 125 upwardly to decrease the coupling between the cylinder 125 and the inductor LT and thus to increase the inductance in the oscillator 1% and to decrease the frequency of operation thereof toward the resonance frequency PR and the frequency of operation in the oscillator 1% will be stabilized at substantially the resonance frequency FR thereof by the action of the coil section 115A and the cylinder T25.

Movement of the rod 128 upwardly also moves the actuator 150 upwardly and permits closure of the switch springs S4 thus restornig the circuit energizing the mag net coil Mil. The magnet M1 therefore again pulls the arm 142 to the left as viewed in FIG. 1 to store energy in the spring 143 and to place the pawl 14's in operative position with respect to the ratchet wheel 135. The mechanism is therefore in condition again to reverse the conditions of the switch springs S1 and S2, if required.

Assuming that the switch springs 81 are closed to place the coil section 115A in the cathode circuit of the amplifying tube 1M so that an increase in plate current will cause a decrease in the operating frequency of the oscillator 100, placement of a load between the capacitor plates CT which has a characteristic load impedance tending to cause operation of the oscillator Hit? at the frequency F2 will cause an undesired operation of the control circuit. More specifically, if the oscillator is operating at the frequency Fll, the current flowing through oanne iii the coil section 115A will tend to move the cylinder 125 upwardly thus decreasing the coupling with the inductor LT to increase the effective inductance thereof and to lower the operating frequency of the oscillator lint Lowering the frequency of operation serves further to increase the plate current through the amplifying tube lliil and thus further to decrease the frequency of operation of the oscillator 103, the process being cumulative and regenerative so that very quickly the oscillator will be operating at the frequency F3 and the plate current through the amplifying tube Till will be the current TF3. The portion of the current 1P3 flowing through the coil section 115A will raise the rod 123 upwardly a distance suilicient to bring the actuator 156 into a position to open the switch springs S3 thereby interrupting the current to the magnet coil 14%. The magnet 141 thereupon releases the armature 142 and the spring 143 acts to pull the arm 137 to the right thereby rotating the ratchet wheel 135 in a counterclockwise direction 45 degrees which in turn rotates the cam lll fi in a counterclockwise direction 45 degrees to place the cam follower 112 upon a cam surface 132 thereby to open the switch springs S1 and to close the switch springs S2. This places the coil section 11513 in the cathode circuit of the amplifying tube 181 and, accordingly, the portion of the current 1P3 flowing therethrough will immediately pull the iron member 136* and the attached rod 228 and cylinder 125 downwardly to decrease the inductance of the inductor LT and I the food being cooked, the power thus to increase the frequency of operation of the oscillator 1th} to substantially the resonance frequency FR thereof. The action of the coil section 1158 and the cylinder will thereafter serve to tend to stabilize the operation of the oscillator 1% at substantially the resonance frequency FR thereof.

A cooking operation is accomplished utilizing the cooking appliance it as follows. The user opens the door section 13 by means of the handle 22 to place the material to be cooked between the capacitor plates CT and within the cooking cavity 14, the opening of the door section 13 serving automatically to ground the capacitor plates CT by means of the grounding devices 44) and to remove the operating potentials of the oscillator 106 by means of the door switch 36. After closing the door section 13, the user will then close the switch- 52 which will apply operating potentials to the oscillator lift The oscillator 10th will begin to operate and assuming that the load impedance of the food between the capacitor plates CT would tend to cause operation thereof at the frequency F1, a portion of the increase in current all?! through the amplifying tube 1M will also flow through the coil sect-ion 1 25B tending to move the iron member 139 downwardly and with it the cylinder 125 to decrease the inductance of the inductor LT and to increase the frequency of operation of the oscillator ltlil to substantially the resonance frequency FR thereof. Cooking energy is removed from the oscillator 1% by absorption in withdrawn from the oscillator 1% being a function of the change in plate current dlPl.

Immediately upon closure of the main switch 52, the magnet coil 1 51 is energized to pull the armature 142 to the left and to store energy in the spring 143 to supply the circuit to reverse the condition of the switch springs S1 and S2, if required. Assuming that the food placed between the capacitor plates CT has a characteristic load impedance that would tend to cause operation of the oscillator 1% at the frequency F2, then the iron member 13% will immediately be pulled downwardly quickly due to the regenerative effect of decreasing the inductance of the inductor LT and increasing the frequency of operation of the oscillator res so that the frequency F i and the change in plate current dlPd would be quickly achieved so that the switch springs 184 would be opened substantially immediately. The magnet coil 149 would be deenergized whereby the spring 143 acting through the pawl 1 l 136 would rotate the cam 131 through 45 degrees to open the switch springs S2 and to close the switch springs S1 thereby placing the coil section USA in the cathode circuit of the amplifying tube fill. The portion of the current 1P3 flowing through the coil section 115A would quickly move the cylinder 125 upwardly to increase the inductance of the inductor LT and to decrease the frequency of the operation of the oscillator tilt) substantially to the resonance frequency FR thereof. The above de scribed cycle of operation would occur substantially instantaneously so that the frequency of operation of the oscillator 16%) would not exceed that prescribed by the Federal Government for high frequency heaters of the type embodied in the cooking appliance 16*.

Similarly if the switch springs S1 were closed to place the coil section USA in the cathode circuit of the amplifying tube 101. when food was placed between the capacitor plates CT that would tend to cause operation of the oscillator 1% at the frequency Fit, the network described above would operate substantially immediately to open the switch springs S1 and to close the switch springs S2 thereby to place the coil section 11513 in the cathode section of the amplifying tube 1M and thus to cause operation of the circuit in a manner to insure that the oscillator 1G0 operates substantially at the resonance frequency FR thereof. It is to be noted that upon the opening of the main switch 52 the condition of the switch springs 81 and S2 is automatically reversed whereby the reversal of the condition thereof may be required even though like foods are placed between the capacitor plates CT in consecutive uses of the cooking appliance it).

In view of the foregoing, it is apparent that there has been provided an improved electronic cooking appliance and particularly a cooking appliance provided with an improved arrangement for automatically adjusting the operating frequency of the oscillator supplying the high frequency cooking power to the resonance frequency thereof to compensate for changes in the characteristic load impedance of the food being cooked. While there has been described what is at present considered to be a preferred embodiment of the invention, it will be understood that various changes and modifications may be made therein, and it is intended to cover in the appended claims all such changes and modifications that fall within the true spirit and scope of the invention.

What is claimed is:

1. In a cooking appliance, metal Wall structure defining a substantially closed cooking cavity, a pair of spacedapart electrodes mounted in said cooking cavity and constituting a capacitor and adapted to receive therebetween the food to be cooked by electrostatic heating thereof, whereby the capacitance of said capacitor is dependent upon the characteristic load impedance of the food being cooked, an inductor, means connecting together said capacitor and said inductor to define a tank circuit, apparatus operatively associated with said inductor and selectively operative to adjust the, inductance thereof, an amplifying device having first and second power terminals and a control terminal, a source of operating potential for said amplifying device, means connecting the terminals of said amplifying device and said tank circuit and said source of potential to define an oscillator having a resonance frequency in the high frequency band that is dependent upon the capacitance of said capacitor and upon the inductance of said inductor and that is characterized by a minimum current through said amplifying device when said oscillator is operating at the resonance frequency thereof and by an increase in the current through said amplifying device when said oscillator is operating at a frequency in the high frequency band other than at the resonance frequency thereof, whereby a change in the characteristic load impedance of the food being cooked is operative to change the capacitance of said capacitor and consequently the operating frequency of said oscillator to increase the current through said amplifying device,

and means responsive to an increase in the current through said amplifying device for operating said apparatus to adjust the inductance of said inductor to a value so as to effect operation of said oscillator substantially at the resonance frequency thereof, thereby to compensate for different characteristic load impedances of the food being cooked.

2. in a cooking appliance, metal wall structure defining a substantially closed cooking cavity, a pair of spacedapart electrodes mounted in said cooking cavity and constituting a capacitor and adapted to receive therebetween the food'to be cooked by electrostatic heating thereof, whereby the capacitance of said capacitor is dependent upon the characteristic load impedance of the food being cooked, an inductor, means connecting together said capacitor and said inductor to define a tank circuit, apparatus operatively associated with said inductor and selectively operative to adjust the inductance thereof, an amplifying device having first and second power terminals and a control terminal, a source of operating potential for said amplifying device, electromagnetic means for operating said apparatus to adjust the inductance of said inductor, and means connecting the terminals of said amplifying device and said tank cirupon the capacitance of said capacitor and upon the inductance of said inductor and that is characterized by a minimum current through said'amplifying device and said electromagnetic means when said oscillator is operating at the resonance frequency thereof and by an increase in the current through said amplifying device and said electromagnetic means when said oscillator is operating at a frequency in the high frequency band other than at the resonance frequency thereof, whereby a change in the characteristic load impedance of the food being cooked is operative to change the capacitance of said capacitor and consequently the operating frequency of said osciliator to increase the current through said amplifying device, said electromagnetic means being responsive to an increase in the current through said amplifying device for operating said apparatus to adjust the inductance of said inductor to a value so as tov effect operation of said oscillator substantially at the resonance frequency thereof, thereby to compensate for different characteristic load impedances of the food being cooked.

3. The cooking appliance set forth in claim 2, wherein said amplifying device is an electron discharge tube includin a cathode'and a plate and a control grid, and said electromagnetic means is a coil in the cathode circuit of said tube.

4. In a cooking appliance, metal wall structure defining a substantially closed cooking cavity, a pair of spaced-apart electrodes mounted in said cooking cavity and constituting a capacitor and adapted to receive therebetween the food to be cooked by electrostatic heating thereof, whereby the capacitance of said capacitor is de pendent upon the characteristic load impedance of the food being cooked, an inductor, means connecting together said capacitor and said inductor to define a tank circuit, an electrically conductive member operatively associated with said inductor and movable toward and away therefrom to adjust the inductance thereof, an amplifying device having first and second power terminals and a control terminal, a source of operating potential for said amplifying device, means connecting the terminals of said amplifying device and said tank circuit and said source of potential to define an oscillator having a resonance frequency in the high frequency band that is dependent upon the capacitance of said capacitor and upon the inductance of said inductor and that is characterized by a minimum current through said amplifying device when said oscillator is operating at the 13 resonance frequency thereof and by an increase in the current through said amplifying device when said oscillator is operating at a frequency in the high frequency band other than at the resonance frequency thereof, whereby a change in the characteristic load impedance of the food being cooked is operative to change the capacitance of said capacitor and consequently the operating frequency of said oscillator to increase the current through said amplifying device, and means responsive to an increase in the current through said amplifying device to move said conductive member to adjust the inductanceof said inductor to a value so as to effect operation of said oscillator substantially at the resonance frequency thereof, thereby to compensate for the different characteristic load impedance of the food being cooked.

5. The cooking appliance set forth in claim 4, wherein said conductive member is a hollow cylinder movable toward and away from said inductor respectively to increase and to decrease the coupling therebetween and respectively to increase and to decrease the frequency of operation of said oscillator.

6. In a cooking appliance, metal wall structure defining a substantially closed cooking cavity, a pair of spaced-apart electrodes mounted in said cooking cavity and constituting a capacitor and adapted to receive therebetween the food to be cooked by electrostatic heating thereof, whereby the capacitance of said capacitor is dependent upon the characteristic load impedance of the food being cooked, an inductor, means connecting together said capacitor and said inductor to define a tank circuit, an electrically conductive non-magnetic member operatively associated with said inductor and movable toward and away therefrom to adjust the inductance thereof, an amplifying device having first and second power terminals and a control terminal, a source of operating potential for said amplifying device, electromagnetic means responsive to the current through said amplifying device, means connecting the terminals of said amplifying device and said tank circuit and said electromagnetic means and said source of potential to define an oscillator having a resonance frequency in the high frequency band that is dependent upon the capacitance of said capacitor and upon the inductance of said inductor and that is characterized by a minimum current through said amplifying device and said electromagnetic means when said oscillator is operating at the resonance frequency thereof and by an increase in the current through said amplifying device and said electromagnetic means when said oscillator is operating at a frequency in the high frequency band other than at the resonance frequency thereof, whereby a change in the characteristic load impedance of the food being cooked is operative to change the capacitance of said capacitor and consequently the operating frequency of said oscillator to increase the current through said amplifying device, and a magnetic armature associated with said electromagnetic means and connected to said electrically conductive member and movable in response to an increase in the current through said electromagnetic means for moving said electrically conductive member to adjust the inductance of said inductor to a value so as to effect operation of said oscillator substantially at the resonance frequency thereof, thereby to compensate for the difi'erent characteristic load impedance of the food being cooked.

7. The cooking appliance set forth in claim 6, wherein the connection between said electrically conductive member and said magnetic armature is adjustable, thereby to permit adjustment thereof to insure operation of said oscillator at the resonance frequency thereof.

8. In a cooking appliance, metal wall structure defining a substantially closed cooking cavity, a pair of spaced-apart electrodes mounted in said cooking cavity and constituting a capacitor and adapted to receive therebetween the food to be cooked by electrostatic heating 14 thereof, whereby the capacitance of said capacitor is dependent upon the characteristic load impedance of the food being cooked, an in motor, means connecting together said capacitor and said inductor to define a tank circuit, apparatus operatively associated with said inductor and selectively operative to adjust the inductance thereof, am amplifying device having first and second power terminals and a control terminal, a source of operating potential for said amplifying device, means connecting the terminals of said amplifying device and said tank circuit and said source of potential to define an oscillator having a resonance frequency in the high frequency band that is dependent upon the capacitance of said capacitor and upon the inductance of said inductor and that is characterized by a minimum current through said amplifying device when said oscillator is operating at the resonance frequencythereof and by an increase in the current through said amplifying device when said oscillator is operating at a frequency in the high frequency band other than at the resonance frequency thereof, whereby a change in the characteristic load impedance of the food being cooked is operative to change the capacitance of said capacitor and consequently the operating frequency of said oscillator to increase the current through said amplifying device, first means responsive to an increase in the current through said amplifying device for operating said apparatus to increase the inductance of said inductor to a value to decrease the frequency of operation of said oscillator substantially to the resonance frequency thereof, second means responsive to an increase in the current through said amplifying device for operatin said apparatus to decrease the inductance of said inductor to a value to increase the frequency of said oscillator substantially to the resonance frequency thereof, and means for selectively operating said first responsive means when the characteristic load impedance of the food to be cooked tends to cause operation of said oscillator at a frequency above the resonance frequency thereof and for selectively operating said second responsive means when the characteristic load impedance of the food to be cooked tends to cause operation of said oscillator at a frequency below the resonance frequency thereof, thereby to compensate for the different characteristic =load impedances of the food being cooked and to insure operation of said oscillator substantially at the resonance frequency thereof.

9. The cooking appiiance set forth in claim 8, wherein said first means and said second means and said means for selectively operating said first and second means are all electromagnetic and are all responsive to the current flowing through said amplifying evice.

10. In a cooking appliance, metal wall structure defining a substantially closed cooking cavity, a pair of spaced-apart electrodes mounted in said cooking cavity and constituting a capacitor and adapted to receive therebetween the food to be cooked by electrostatic heating thereof, whereby the capacitance of said capacitor is dependent upon the characteristic load impedance of the food being cooked, an inductor, means connecting together said capacitor and said inductor to define a tank circuit, an electrically conductive member operatively associated with said inductor and movable toward and away therefrom to adjust the inductance thereof, an amplifying device having first and second power terminals and a control terminal, means connecting the terminals of said amplifying device and said tank circuit and said source of potential to define an oscillator having a reso nance frequency in the high frequency band that is dependent upon the capacitance of said capacitor and upon the inductance of said inductor and that is characterized by a minimum current through said amplifying device when said oscillator is operating at the resonance frequency thereof and by an increase in the current through said amplifying device when said oscillator is operating at a frequency in the high frequency band other than at the resonance frequency thereof, whereby a change in the characteristic load impedance of the food being cooked is operative to change the capacitance of said capacitor and consequently the operating frequency of said oscillator to increase the current through said amplifying device, first means responsive to an increase in the current through said amplifying device to move said conductive member away from said inductor to increase the inductance thereof to a value to decrease the frequency of operation of said oscillator substantially to the resonance frequency thereof, second means responsive to an increase in the current through said amplifying device to move said conductive member toward said inductor to decrease the inductance thereof to a value to increase the frequency of said oscillator substantially to the resonance frequency thereof, and means for selectively operating said first responsive means when the characteristic load impedance of the food to be cooked tends to cause operation of said oscillator at a frequency above the resonance frequency thereof and for selectively operating sa-id second responsive means when the characteristic load impedance of the food to be cooked tends to cause operation of said oscillator at a frequency below the resonance frequency thereof, thereby to compensate for the different characteristic load impedance of the food being cooked and to insure operation of said oscillator substantially at the resonance frequency thereof.

11. In a cooking appliance, metal wall structure defining a substantially closed cooking cavity, a pair of spacedapart electrodes mounted in said cooking cavity and constituting a capacitor and adapted to receive therebetween the food to be cooked by electrostatic heating thereof, whereby the capacitance of said capacitor is dependent upon the characteristic load impedance of the food being cooked, an inductor, means connecting together said capacitor and said inductor to define a tank circuit, an electrically conductive member operatively associated with said inductor and movable toward and away therefrom to adjust the inductance thereof, an amplifying device having first and second power terminals and a control terminal, a source of operating potential for said amplifying'device, a first electromagnetic means respon sive to the current through said amplifying device, second electromagnetic means responsive to the current through said amplifying device, means connecting the terminals of said amplifying device and said tank circuitand a selected one of said electromagnetic means and said source of potential to define an oscillator having a resonance frequency in the high frequency band that is dependent upon the capacitance of 'said capacitor and upon the inductance of said inductor and that is characterized by a minimum current through said amplifying device and said selected electromagnetic means when said oscillator is operating at the resonance frequency thereof and by an increase in the current through said 16 amplifying device and said selected electromagnetic means when said oscillator is operating at a frequency in the high frequency band other than at the resonance frequency thereof, a magnetic armature associated with said electromangetic means and connected to said electrically conductive member and movable in response to operation of said first electromagnetic means for moving said electrically conductive member away from said inductor to increase the inductance thereof and to decrease the frequency of operation of said oscillator substantially to the resonance frequency thereof and movable in response to operation of said second electromagnetic means for moving said electrically conductive member towards said inductor to decrease the inductance'thereof and to increase the frequency of operation of said oscillator substantially to the resonance frequency thereof, and means responsive to the current through said amplifying device for selecting the one of said electromagnetic means to be connected in circuit with said amplifying device so as to effect operation of said oscillatorsubstantially at the resonance frequency thereof thereby to compensate for different characteristic load impedances of the food being cooked.

12. The cooking appliance set forth in claim 11, wherein saidelectromagnetic means are coils, and said means for selecting one of said electromagnetic means to be connected in circuit with said amplifying device comprises a first switch having a first position connecting a first of said coils in circuit with said amplifying device and a second position connecting a second of said coils in circuit with said amplifying device, and asecond switch operable automatically when the current through said amplifying device reaches a predetermined value to operate said first switch from one of its positions to the other.

13. The cooking appliance set forth in claim 11, wherein said electrically conductive member is a hollow cylinder, and said hollow cylinder and said magnetic member are interconnected by an electrical insulating support.

-14. The cooking appliance set forth in claim 13, wherein the position of said cylinder upon said rod is adjustable and the portion of the current through said amplifying device which flows through said electromagnetic means is adjustable, thereby to adjust said circuit to insure operation of said oscillator at the resonance frequency thereof.

References Cited in the file ofthis patent UNITED STATES PATENTS 2,453,529 Mittelmann, .Nov. 9, 1948 2,588,304 Storm Mar. 4, 1952 2,705,286 7 Kinn Mar. 29, 1955 2,780,705 Siegmeth Feb. 5, 1957 FoReio PATENTS 623,000 Great Britain May 11, 1949 646,013 Great Britain Nov. 15, 1950 

1. IN A COOKING APPLIANCE, METAL WALL STRUCTURE DEFINING A SUBSTANTIALLY CLOSED COOKING CAVITY, A PAIR OF SPACEDAPART ELECTRODES MOUNTED IN SAID COOKING CAVITY AND CONSTITUTING A CAPACITOR AND ADAPTED TO RECEIVE THEREBETWEEN THE FOOD TO BE COOKED BY ELECTROSTATIC HEATING THEREOF, WHEREBY THE CAPACITANCE OF SAID CAPACITOR IS DEPENDENT UPON THE CHARACTERISTIC LOAD IMPEDANCE OF THE FOOD BEING COOKED, AN INDUCTOR, MEANS CONNECTING TOGETHER SAID CAPACITOR AND SAID INDUCTOR TO DEFINE A TANK CIRCUIT, APPARATUS OPERATIVELY ASSOCIATED WITH SAID INDUCTOR AND SELECTIVELY OPERATIVE TO ADJUST THE INDUCTANCE THEREOF, AN AMPLIFYING DEVICE HAVING FIRST AND SECOND POWER TERMINALS AND A CONTROL TERMINAL, A SOURCE OF OPERATING POTENTIAL FOR SAID AMPLIFYING DEVICE, MEANS CONNECTING THE TERMINALS OF SAID AMPLIFYING DEVICE AND SAID TANK CIRCUIT AND SAID SOURCE OF POTENTIAL TO DEFINE AN OSCILLATOR HAVING A RESONANCE FREQUENCY IN THE HIGH FREQUENCY BAND THAT IS DEPENDENT UPON THE CAPACITANCE OF SAID CAPACITOR AND UPON THE INDUCTANCE OF SAID INDUCTOR AND THAT IS CHARACTERIZED BY A MINIMUM CURRENT THROUGH SAID AMPLIFYING DEVICE WHEN SAID OSCILLATOR IS OPERATING AT THE RESONANCE FREQUENCY THEREOF AND BY AN INCREASE IN THE CURRENT THROUGH SAID AMPLIFYING DEVICE WHEN SAID OSCILLATOR IS OPERATING AT A FREQUENCY IN THE HIGH FREQUENCY BAND OTHER THAN AT THE RESONANCE FREQUENCY THEREOF, WHEREBY A CHANGE IN THE CHARACTERISTIC LOAD IMPEDANCE OF THE FOOD BEING COOKED IS OPERATIVE TO CHANGE THE CAPACITANCE OF SAID CAPACITOR AND CONSEQUENTLY THE OPERATING FREQUENCY OF SAID OSCILLATOR TO INCREASE THE CURRENT THROUGH SAID AMPLIFYING DEVICE, AND MEANS RESPONSIVE TO AN INCREASE IN THE CURRENT THROUGH SAID AMPLIFYING DEVICE FOR OPERATING SAID APPARATUS TO ADJUST THE INDUCTANCE OF SAID INDUCTOR TO A VALUE SO AS TO EFFECT OPERATION OF SAID OSCILLATOR SUBSTANTIALLY AT THE RESONANCE FREQUENCY THEREOF, THEREBY TO COMPENSATE FOR DIFFERENT CHARACTERISTIC LOAD IMPEDANCES OF THE FOOD BEING COOKED. 